spotify: resilient fetching, progress bars, venv launcher

- fetch() uses a retrying session (connection resets, 5xx, 429 Retry-After)
  with backoff, so a transient drop mid-fetch no longer aborts the run.
- tqdm progress bars on all paginated fetches (driven by paging `total`).
- Flow.fetch_spotify(): drop stray `self` from the @staticmethod.
- gui.sh runs the venv's streamlit from the script dir; add requirements.txt.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gui.sh

@@ -1 +1,5 @@
-streamlit run Gui.py
+#!/usr/bin/env bash
+# Run from the script's directory using the project virtualenv,
+# so it works regardless of whether the venv is activated / on PATH.
+cd "$(dirname "$0")"
+exec .venv/bin/streamlit run Gui.py "$@"